This invention relates to a scroll compressor and a refrigerator incorporating the scroll compressor and, more particularly, to a scroll type refrigerator capable of operating efficiently at low temperatures.
In low-temperature refrigerators, as is well known, the suction pressure is reduced if the evaporation temperature decreases. The compression ratio is accordingly increased and the volumetric efficiency of the compressor is thereby reduced so that the refrigerating capacity becomes smaller. The compression efficiency is also reduced, the desired power is increased and the temperature of the discharged gas becomes considerably high. As a result, the lubricating oil deteriorates and, in the case of a sealed type compressor, there is the problem of deterioration in the insulating properties of the incorporated electric motor.
A two-stage compression system has therefore been adopted in which the compressing process is divided into two stages to compensate for these drawbacks at evaporation temperatures of -45.degree. to -70.degree. C., at which the tendency to such a result is marked. Coventionally, a volume type compressor such as a reciprocating compressor or a screw compressor is used as a two-stage compressor constituting such a two-stage compression system. A two stage compression-one stage expansion type refrigerator is used as a typical two-stage compression system.
The two-stage compression-one stage expansion cycle is also applied to refrigeration in the range of evaporation temperatures ordinarily attainable by single-stage compression, because the refrigerating capacity of this cycle can be increased by supercooling of high-pressure refrigerant liquid to increase the coefficient of performance. For example, Japanese Patent Unexamined Publication No. 49-54943 discloses a refrigerator in which the gas is injected during compression by using a screw compressor so that the high-pressure refrigerant liquid is supercooled by the effect of this injection. Also, Japanese Patent Unexamined Publication No. 57-76289 discloses a refrigerator using a scroll compressor, wherein gas injection is effected for energy saving and for increasing the capacity at the time of cooling and heating.
If a two-stage compressor is used, low temperatures of -45.degree. to -70.degree. C. can be obtained but the two-stage compressor requires two sets of compression mechanism units and motor units for driving the compression mechanism or the mechanism for two-stage compression must be complicated, resulting in an increase in manufacture cost. Two-stage compressor is not practically applicable to small-capacity refrigerators because of the problem of its complicated mechanism and the increase in manufacture cost.
On the other hand, it can be presupposed that screw or scroll compressors can be realized which are capable of operating at a high volumetric efficiency and at a high compression efficiency even when the compression ratio is high because, in screw or scroll compressors, the compressed gas leakage thereof during compression is small even under a high compression ratio condition as can be understood from the compression principle of these compressors. However, screw or scroll compressors have not been put to practical use for the reasons described below. Details of a geometrical theory relating to the theory of compression using a scroll compressor have been reported in "Geometrical Theory of Scroll Compressors" by Morishita et al., Turbo Machine (Turbo Kikai) No. 4, Volume 13, April, 1985. In this report are described the relationship between the theoretical built-in volume ratio (hereinafter referred to as "set volume ratio") and the number of turns of the voluted body (hereinafter referred to as "wrap"), the set volume ratio, the optimum compression ratio, and unnecessary power consumed when the operating condition deviates from the optimum compression ratio. In the case of a scroll compressor, the set volume ratio is determined from the compression ratio at which the scroll compressor ordinarily operates and from the geometric theory of the scroll compressor so that the optimum compression ratio is closer to the compression ratio at which the compressor ordinarily operates.
It can be theoretically presupposed that scroll compressors are suitable for a high compression ratio compressor from the fact that in scroll compressors the confining capacity can be 100% compressed for discharge in theory, and the fact that some intermediate compression chambers are formed during the period between suction and discharge and that the number of intermediate chambers is increased as the set volume capacity is increased so that the leakage of the compressed fluid becomes smaller. However, in a case where a scroll compressor is designed for a refrigerator operating at evaporation temperatures of -45.degree. to -70.degree. C. with Freon 22 used as a refrigerant, and if the condensation temperature is 40.degree. C., the compression ratio is about 20 when the evaporation temperature is -45.degree. C., or is about 75 when the evaporation temperature is -75.degree. C. To set the optimum compression ratio in this range, it is necessary to select a set volume ratio in a range of 12 to 38. If the geometrical shape of the laps is determined from a set volume ratio of 25 which is the mean value of the range of 12 to 38, the number of wrap turns is about 20.
This number is 5 to 10 times larger than the number of lap turns in the conventional scroll compressors put to practical use, which is about 2 to 4. In this case, the overall size of the compressor is very large, as can be understood from the fact that the outside size of the voluted body is generally proportional to the number of turns thereof. The mass production technique for working such a large voluted body with accuracy must be improved to a very high level.
Thus, it is not possible to obtain low temperatures determined by evaporation temperatures of -45.degree. to -70.degree. C. by using scroll compressors in practice. For these reasons, two-stage compression type compressors have conventionally been used.